ODA Course 9.6.

The neural
magnocellular basis of
dyslexia
Supported by The Dyslexia Research Trust (www.dyslexic.org.uk),
Dyers & Colourists, Esmee Fairbairn, Garfield Weston and Wellcome
Trusts, BBC Children in Need
Reading is difficult!

Reading is a painful task.
It extinguishes the light from the
eyes.
It bends the back.
It crushes the viscera and the ribs.
It brings forth pain to the kidneys
and weariness to the whole body.
13th C. Florentine monk or 20th C dyslexic!
Reading is difficult because it
requires:
1. Rapid visual identification of letters and
their order; even in experienced good readers this
process is rate limiting
2. Rapid auditory translation into the sounds
they stand for
3. Background knowledge of phonology- how
words can be split down into separate phonemes
All these processes may depend on the timing
properties of magnocellular neurones
Reading is
difficult! Prevalence of Dyslexia
1 in 3 of US & UK 11yr
olds leave primary
school unable to read.
1 in 3 of US & UK
adults leave high
school effectively
illiterate. girls boys
Reading failure is the
commonest cause of
childhood misery,
depression, even
suicide,
OR frustration,
aggression, crime;
75% of those in gaol
are illiterate.
Commonest disability
among College
students boys dyslexic boys dyslexic girls girls
Developmental
dyslexia
1898 Pringle Morgan 1980s Linguistic theory -
‘Word blindness’ phonological deficit
1930s Samuel Orton - 1990s Development of the
strephosymbolia brain is different:
1950s McDonald Critchley planum temporale,
Parietal lobe ectopias, problems with
1960s Social denigration - all kinds of timing and
“Middle class children are sequencing
dyslexic,working class 2000s Magnocellular theory
children are thick!” impaired development
of visual, auditory and
motor magno-neurones
What is Developmental
Dyslexia?
Reading and spelling significantly below that expected
from subject’s age and intelligence, despite good
health, teaching and cultural experience
Symptoms History

“Middle class children are called ‘dyslexic’, where
working class children are called stupid”

Has dyslexia a real neurobiological
basis or is it just a middle class
invention?
Biological or invented?
• Association with other
• Genetic basis – family neurodevelopmental
history, gender disorders (ADHD, dyspraxia,
SLI, autism spectrum)
• Brain differences – • Often difficult birth
ectopias, smaller m- • Delayed crawling, walking,
cells, cerebellum, fMRI speaking
• Glue ear when learning to
• Unstable visual & speak – impaired auditory
auditory attention - perception - speech
impairments: lisps,
poor eye control & spoonerisms
pronunciation • Association with
autoimmunity
2nd trimester
ectopias in
dyslexic brain.
Seen also in
auto immune
mice
Leftareas
Brain hemisphere language
activated less
inareas that when
dyslexics activate less in
reading
dyslexics
Many children
complain of
visual difficulties
with reading.
Often their eyes
wobble when
they try to read
This may be due
to weak visual
magnocellular
function
Reading is primarily a visual
process
Visual processing
When reading the eyes have to
scan the text. Words are only
identified during very brief ‘fixations’
that last only 1/3rd sec. Eyes must
be held steady during these
fixations. Otherwise letters can
appear to move around and jump
over each other
Retinal Ganglion
cells

Dyslexic Control
Homophone test of
visual/orthographic rain rane
skill
“Which one is the
proper spelling?”

This task can’t be
solved by sounding out
the letters.

Best correlate
of reading skill
even in adults
Motion
21%
Other
79%

Not a diagnostic test, but over 20% of individual differences in
orthographic reading ability (good readers & dyslexics) can be
explained just by their low level visual magnocellular
sensitivity, independent of their cognitive skills.
Thus low level visual magnocellular sensitivity helps determine
how well orthographic skills develop.
The visual magnocellular system
stabilises the eyes to avoid visual
wobble
Unwanted Feedback to
Detected by M-
image motion, system
eye muscle
‘retinal slip’ control system

0.00
low high
visual motion sensitivity
Vergence control
• The eyes have to converge
for near vision when reading
• Control of vergence eye
movements is dominated by
the visual magno system
• The vergence eye movement
control system is the most
vulnerable to drugs and
disease
• Dyslexics have very
unstable vergence control
Magnocellular processing sharpens:

into
Interventions that improve
magno- function and eye
control often improve
reading
In some 9 yr old children with
poor binocular control
temporary blurring of left
eye improves vergence
control and reading by 2
m/m (RCT)
In older children exercises
can stabilise binocular
fixation and greatly
improve reading (RCT)
Blue or yellow coloured filters
(RCT) can rebalance input
to the visual M- system
Blue or yellow filters can help some
children to keep the letters still
Coloured Filters
• Although they do not contribute to colour
vision, retinal magnocellular ganglion
cells are most responsive to yellow light
• Many are inhibited by red
• Blue filters reduce red input, so in some
children blue can increase magnocellular
sensitivity - visual motion, binocular
control and reading
• Other magnocells are inhibited by blue.
Hence in other children yellow filters can
improve m- function and reading
Yellow Glasses
Before Only 1 week later
Elucidating the role of the visual magnocellular system in
reading has enabled us to develop techniques for
helping 75% of the dyslexics we see

patching
Magno deficit causes many dyslexics to confuse the
visual order of letters!
The visual/orthographic and
auditory/phonological pathways
2nd and 3rd formants
ascend in frequency
for ‘b’;
but descend for ‘d’.
Subtle auditory
impairments may
reduce sensitivity to
these changes in
sound frequency
Meltham Primary School
Measure sensitivity to
the frequency changes
that signal different letter
sounds
Developmental Dyslexics are less sensitive to
changes in sound frequency and intensity.
• Slow frequency changes in speech are tracked in real time by
large magnocells in the auditory system

Thus the most important determinant of overall reading
ability appears to be low level magnocellular sensitivity.
Encouraging because this can be improved by training
The magnocellular systems also project
strongly to the cerebellum – the brain’s
autopilot - a magnocellular structure

Cerebellum
Control Head movement Dyslexic
balancing on one leg poor balance
The Cerebellum & balance
• The cerebellum is the brain’s autopilot for
timing, motor prediction and accurate motor
programming: balance and skilled movements
• Magnocellular systems all project to the
cerebellum and the cerebellum is part of the
magno network
• Cerebellum is underactive in many dyslexics
• Explains their coordination problems, and may
contribute to their reading difficulties
• But this is not their only cause; expensive
balance exercises may improve balance but
not reading
Sensorimotor Basis of
Dyslexia
• Low visual magnocellular sensitivity
orthographic weakness
• Low auditory magnocellular
sensitivity phonological problems
• Lower kinaesthetic magnocellular
sensitivity
• Lower motor magnocellular
sensitivity incoordination, poor
balance
What causes this
general
magnocellular
impairment?